Non-work is one of the most essential pieces of the puzzle
Each position represents a pre-determined capacity of work. Once that position is filled with an employee, that capacity is reduced by the constraints they have (vacation, breaks, etc).
Therefore, a planner must schedule the non-work first and remove the work capacity first before trying to assign the work. Although this seems to be in reverse order of what someone should do, it is essential to start with this step. Training, vacation, holidays, meetings and all other types of events that do not contribute to the core workload need to be scheduled first.
While placing this reduction of capacity, the planner can keep an eye on just that: the remaining capacity. On every day of every week, there is a potential of work assignments that remain and that are not removed. If the planner can keep daily and weekly capacity totals above the required workload, then the planner may have an easier job when distributing the actual work afterwards (I did say may since the individual constraints could prevent some shift assignments).
It is in this step that you must involve the employees as often and as transparently as possible. You need to gather the information about time off early and often. You need to establish deadlines (reasonable ones) for the employees to make their requests. You need to remind them often of the upcoming deadlines. Etc.
If you don’t involve your employees or don’t ask, they will only ask once they see the schedule and see that you scheduled them on a day they wanted off. Then the planner won’t be able to give them the day off. Then the employee will be pissed because they never ask for anything and never miss work and that’s how they are treated. Then when that day comes, that employee will call in sick. Then the planner will have to make a hasty decision and call someone on overtime. All this because you didn’t ask early enough…
The point in this step is that non-work is one of the most essential pieces of the puzzle because everyone is concerned about unproductive time (management because of the cost, employees because it’s their time off, etc). Keeping an eye on capacity early and often is key.